Typical gaseous reduction systems incorporating vertical shaft, moving bed iron ore reduction reactors are disclosed in U.S. Pat. Nos. 3,765,872; 3,770,421; 3,779,741; and 3,816,102. In such systems reduction of the ore has commonly been effected by a reducing gas largely composed of carbon monoxide and hydrogen. Such systems typically comprise a vertical shaft reactor having a reducing zone in the lower portion thereof and a cooling zone in the lower portion thereof. The ore to be reduced is fed to the top of the reactor and flows downwardly therethrough, first through the reducing zone wherein it is brought into contact with heated reducing gas from the reformer and then through the cooling zone wherein the reduced ore is cooled before being removed at the bottom of the reactor. Effluent gas from the reducing zone is cooled to remove water therefrom and in most cases a major part of the cooled effluent gas is reheated and recycled directly or indirectly to the reducing zone. Similarly, at least a part of the coolant gas withdrawn from the cooling zone is usually cooled and recycled to the cooling zone. At its lower end the reactor is provided with some means for controlling the discharge of the cooled sponge iron from the reactor, e.g. a rotary discharge valve, a vibratory chute, conveyor belt or the like.
One of the problems that is encountered in the operation of such moving bed reduction reactors arises out of the tendency of the freshly reduced hot sponge iron pellets to sinter and/or agglomerate into aggregates that obstruct both the smooth downward flow of solid particles through the reactor and the uniform distribution over the cross section of the reactor of the reducing gases flowing upwardly therein. In general, the rate at which the reduction reaction proceeds is a function of temperature. By increasing the reducing gas inlet temperature the rate of reduction can be increased, the residence time of the iron-bearing material within the reduction zone decreased and the productivity of the reactor thereby increased. On the other hand, an increase in the reducing gas temperature increases the tendency of the sponge iron particles to soften, become sticky and to agglomerate. This agglomerating tendency operates to limit the gas temperature that can be used. The particles at the reactor wall are subjected directly to the hottest portion of the reducing gas as it is injected through nozzles or other means through the walls of the reactor into the reducing zone. This results in the particles at the wall being hotter and more likely to become sticky relative to the rest of the descending particles, so that there is a tendency for the particles adjacent to the wall to adhere to the reactor's wall and cause additional obstruction.
The agglomeration problem has been aggravated in recent years by the practice of using relatively high grade iron ore in the form of pellets, rather than lumps of ore of widely varying sizes. Such pellets have greater physical strength and less tendency to disintegrate and pulverize than ores in lump form. Also, with their relatively high iron content produce a lesser amount of gangue when product sponge iron is used in the subsequent steel-making process. On the other hand, because of their higher iron content, such pellets show a greater tendency to agglomerate.
Various solutions to the agglomeration problem have been previously proposed. The simplest and most direct proposal is to put breaker mechanisms having any of various configurations inside the reactor and arranged so that they can be manipulated from outside the reactor to physically break-up the aggregates formed. See U.S. Pat. Nos. 4,449,671 (issued May 22, 1984) and 4,118,017 (issued Oct. 3, 1978). However, these are an added expense, and any such braker positioned within the reactor is itself an obstruction to the free flow of solid particles therethrough. It has been proposed that the ore to be charged to the reactor to be mixed with an inert material having lumps or particles of irregular shape, but the use of such an inert material makes necessary the additional step of separating the product sponge iron from the inert material. Accordingly, neither of these proposed solutions has been found economically satisfactory. An alternative method to reduce agglomeration has been to utilze natural gas addition to a closed cooling loop so that a small proportion of the methane-containing cooling gas flows upwardly to react endothermically with the water and carbon dioxide present in the reducing gas injected into the bottom of the reduction zone. By thus dropping the temperature of the sponge iron rapidly after leaving the reducing zone, the time available for forming agglomerations is lessened. See U.S. Pat. No. 4,556,417.
U.S. Pat. No. 4,002,422 discloses an apparatus for treating particulate iron ore with hot reducing gases in a reducing zone wherein a "cold process gas" is injected to the lower intermediate zone of the reactor. However, there is no teaching of agglomeration problems. Also, the injected gas is disclosed as flowing upwardly through said intermediate zone, being heated by the descending burden and contributing to the process in the same manner as the hot gas fed to the upper zone. If too much natural gas is injected to flow upwardly, this will tend to concentrate in the central core of the iron ore bed in the reducing zone.
U.S. Pat. No. 4,253,867, like U.S. Pat. No. 4,002,422, discloses the introduction of a separate gas stream into the intermediate zone of a reactor. In U.S. Pat. No. 4,253,867 the separate gas stream is a heated mixture of methane and steam and the gas stream flows upwardly rather than downwardly in the intermediate zone causing that core to be at a lower temperature and subjected to a lower quality reducing gas (due to the higher concentration of CH.sub.4) than the rest of the bed. This results in a non-uniform product output.
U.S. Pat. No. 4,054,444 discloses a method for controlling carburization of metallic iron pellets by injecting cold natural gas to the intermediate zone or to the cooling zone. The objective of this natural gas is to increase the carbon content of iron pellets (not to prevent agglomeration). As shown in the patent drawings, the injected natural gas is intended to flow upwardly and to contribute to the reduction of the iron ore.